Methods of making high critical temperature superconductors are known that comprise: starting from the superconductive oxide in powder form, making compacted sintered pellets therefrom, melting the surface of such pellets by means of a laser beam, rapidly cooling the molten layer, and then subjecting it to annealing. Such methods are described in the following articles:
"Concentration of current to the surface and modification by CO.sub.2 laser for oxide superconductor", by H. Nomura et al.; 2nd International Symposium on Superconductivity, November 1989, Taukuba (pp. 423-426);
"The microstructure of superconductivity of YBa.sub.2 CuO.sub.7-x rapidly solidified by a pulse laser" by J. C. Huang et al.; Supercond. Sci. Technol. 1, 1988 (pp. 110-112); and
"Laser zone melting of Bi.sub.2 Sr.sub.2 CaCuO.sub.x superconductors" by K. Ishige et al.; 2nd International Symposium on Superconductivity, November, 1989, Taukuba (pp. 321-324).
The experimental parameters disclosed in those documents do not make it possible to obtain texturing with grains that are elongate in the same direction. In addition, all of the treatments described are performed on pellets having a thickness of greater than one millimeter; this is totally incompatible with making a flexible superconductor.
The essential difficulty is that a superconductive ceramic is a fragile material whose mechanical properties make it very difficult to fabricate in the form of a flexible conductor, particularly in long lengths.
To mitigate these problems of brittleness, attempts have been made to associate the ceramic with a metal support.
For example, metal-clad wires are made in monofilament or multifilament form. That method suffers from the drawback of requiring numerous mechanical forming steps, in particular wire-drawing steps, and intermediate annealing steps. Furthermore, there is a high risk of reaction between the ceramic and the cladding, and this can damage superconductivity given the very small diameter of the ceramic wire.
In another embodiment described in patent application JP-A-2 257 527, the ceramic is deposited as a very thin layer around a metal wire, by wetting the wire as it passes through a bath of Y-Ba-Cu-O type molten oxide. In order to ensure that the superconductive phase is formed on the wire on leaving the bath, the displacement speed must be very low, thereby giving rise to unacceptable reaction between the bath and the material of the wire.
A much more promising technique consists in depositing the ceramic in the form of a thick layer (several hundreds of microns thick) on a flexible metal tape.
Various methods have already been proposed for obtaining thick layers of superconductive ceramic: strip casting; silk-screen printing; calendering; electric arc spraying; and flame spraying. The resulting layers are porous to a greater or lesser extent, and they are crystallized to a greater or lesser extent depending on the method used. Thereafter they are annealed. Methods of that type and implementing plasmas are described in the following documents:
"High Tc superconducting films of YBaCu oxides prepared by low pressure plasma spraying" by K. Tachikawa et al.; Appl. Phys. Lett., 52 (12), 1988; and
"Formation of YBaCuO thick films by plasma spraying" by Y. Wadayama et al.; 2nd International Symposium on Superconductivity, November, 1989, Tsukuba.
The maximum current densities obtained using those two methods are no more than 690 A/cm.sup.2 and 1120 A/cm.sup.2 respectively, and that is not enough.
Finally, proposals have been made in patent application JP-A-63 292530 and in French patent FR-A-2 647 266 for a method in which a precursor of superconductive material is placed in powder form on a substrate, and is then heated by a laser beam so as to make a layer that adheres to the substrate. Bonding can occur only if the laser beam acts throughout the thickness of the precursor powder, thereby likewise acting on the substrate, and this is particularly troublesome when the substrate is a flexible material that is a few tenths of a millimeter thick. In another variant, the powder is sprayed onto the substrate through the laser beam.
That technique does not achieve texturing of the superconductive layer, either. Furthermore, since the concentration by volume of the powder material as deposited on the substrate cannot exceed 30%, the densification that results from the laser treatment gives rise to very large stresses in said material, which stresses are released by cracking, thereby opposing the flow of current.
An object of the present invention is to implement a method of making a flexible superconductor, which method gives rise to a textured material capable of transporting current densities that are much greater than those of known flexible conductors.